Nine  Years’  Operation 

of  the 

Baltimore  Sewage- Works 


BY 

THEODORE  C.  SCHAETZLE 

Principal  Sanitary  Chemist 


REPRINTED  FROM  THE  ENGINEERING 
NEWS-RECORD,  JULY  14  AND  21,  1921 


AUGUST  E.  CHRISTHILF 
Highways  Engineer 

MILTON  J.  RUARK 
Div.  Engineer  ot  Sewers 


Printed  by  The  Highways  Department 
of  the  City  of  Baltimore. 


■%a.n  In 

Co 

NINE  YEARS’  OPERATION 

OF  THE 

BALTIMORE  SEWAGE-WORKS 

BY 

THEODORE  C.  SCHAETZLE 

Principal  Sanitary  Chemist  of  Sewage-Works 
BALTIMORE,  MD. 


WITH  the  publication  of  the  1914  report  of  the 
Baltimore  Sewerage  Commission  the  last  con¬ 
sistent  record  of  the  development  of  Balti¬ 
more's  vast  sewerage  system  ended.  Throughout  the 
years  that  this  report  was  published  some  mention 
was  made  of  the  sewage-works,  always  in  reference 
to  its  construction  features — except  for  the  1911  re¬ 
port,  which  dealt  extensively  with  the  experi¬ 
ments  upon  the  results  of  which  the  present  system 
was  adopted.  Numerous  articles  on  the  construc¬ 
tion  of  the  plant  appeared  in  the  engineering 
periodicals  but  little  as  to  its  operation,  and  some 
erroneous  ideas  regarding  the  Imhoff  tanks  complet¬ 
ed  in  1916  have  been  circulated.  These  facts  com- 


Imhoff  tank 
I  sornot  beoh. 

t&fmhoff  tanks. 

Mgttrhouse.  , 

Outfalh^  \  t 
sev^r  L 

Bar  screens 

Vtushinc 
veafer  mnk 


Revolving  screens  2setflin^^  Pov^er 
"ontro!  buildina  basins  \  house 


Sludqe’ 
_  . .  y  digestion 

fiushtnc^^^ 


1 

1  m 

lifer  beols,3C 
1  1  1 

CIC 

n 

res 

Ij 

{Br 

•)ke^  S 

_ 

1 

1 

2  v^oodsta^ 
pipes  extending 
2500ft  info  nver 


'Sludge  beds 


^  <.'1,  o'  200' 400' 600' 800' 
Sludge  beds  i  t  i  i  \ 


/  dudge 

Sludge  beds  — ->tllllllllllllllllll  digestion 


tanks 


Sludge  drying  plant-  >C3 


FIG.  1.  GENERAL  PLAN  OF  THE  BALTIMORE 

SEWAGE-WORKS. 


bined  with  numerous  requests  for  information  have 
led  me  to  prepare  the  following  resum^  of  the  op¬ 
eration  of  the  Baltimore  sewage-works  from  their 


790427 


2  NINE  YEARS’  OPERATION  OF  THE 

first  full  year  of  use  in  1912  to  the  close  of  1920. 
To  make  the  account  more  readily  understandable 
an  outline  description  of  the  plant  will  first  be 
given. 

On  April  7,  1904,  the  Maryland  Legislature  passed 
the  sewerage  enabling  act  which  stated  that  the 
Sewerage  Commission  of  Baltimore  had  no  auth¬ 
ority  to  construct  any  sewerage  system  which  would 
discharge  sewage,  as  distinguished  from  storm 
water  or  ground  drainage,  into  the  Chesapeake 
Bay  or  any  of  its  tributaries.  Accordingly  the 
commission  ^‘resolved,  that  the  effluent  proposed  to 
be  discharged  into  the  Chesapeake  Bay  or  its 
tributaries  in  the  system  to  be  recommended  by 
the  engineers  shall  be  of  the  highest  practicable  de¬ 
gree  of  purity.^' 

To  meet  these  requirements  the  Back  River  Dis¬ 
posal  Works  were  constructed — and  put  into  oper¬ 
ation  on  Oct.  30,  1911.  Additions  have  been  made 
from  time  to  time  to  care  for  the  increasing  sew¬ 
age  flow  and  at  the  present  writing  further  ex¬ 
tensions  are  under  contract. 

Description  of  Works — The  existing  plant  con¬ 
sists  of  preliminary  vertical  bar  screens  spaced  1 
in.  apart;  a  meter  house  with  five  42  x  21-in.  ven¬ 
turi  meters;  3  preliminary  plain  sedimentation 
tanks,  known  as  hydrolytic  tanks;  28  radial-flow 
Imhoff  tanks,  19  separate  sludge  digestion  tanks  and 
7.8  acres  of  sludge  drying  beds;  4  cylindrical  re¬ 
volving  screens,  trickling  filter  control  house,  30 
acres  of  trickling  filters,  4  small  pump-houses, 
each  equipped  with  an  electrically-driven  centrifu¬ 
gal  pump,  a  hydro-electric  power  house  generating 
power  from  available  head  in  the  treated  sewage, 
and  a  final  effluent  conduit  discharging  into  Back 
River. 

The  hydrolytic  tanks  are  divided  into  two  com¬ 
partments  by  a  curtain  wall,  the  first  compartment 
being  103  ft.  11  in.  long  by  101  ft.  9  in.  wide  and 
10%  ft.  deep,  while  the  larger  or  secondary  com¬ 
partment  is  313  ft.  9  in.  long  by  101  ft.  6  in.  wide 


BALTIMORE  SEWAGE  WORKS  3 

and  13^  ft.  deep.  Both  compartments  have  sloping 
bottoms. 

The  Imhoff  tanks  are  circular.  They  were  orig¬ 
inally  of  the  downward  and  outward  radial-flow 
type,  but  some  have  since  been  converted  to  a 
radial  horizontal  flow.  All  of  them  are  to  be  con¬ 
verted  in  this  manner.  These  tanks  are  39  ft.  inside 
diameter,  13^  ft.  deep  to  the  bottom  of  the  sedi¬ 
mentation  chamber,  and  27  ft.  total  depth. 
The  sludge  chamber  is  23  ft.  in  diameter,  with  a 
conical  bottom  5.3  ft.  deep.  The  capacity  of  the 
sedimentation  chamber  is  8,920  cu  ft.,  and  of  the 
sludge  chamber  3,663  cubic  feet. 

Of  the  19  separate  sludge  digestion  tanks  three 
are  140  x  103  ft.  2  in.  in  plan  and  13  ft.  in  depth 
and  16  are  cylindrical  tanks,  38  ft.  inside  di¬ 
ameter,  15  ft  deep  to  the  conical  bottom,  which  is 
9^  ft.  deep,  making  a  total  depth  of  24%  ft. 

Each  revolving  screen,  open  at  one  end  only,  is 
12  ft.  in  diameter  and  10  ft.  long,  weighs  approxi¬ 
mately  1  %  tons  and  is  constructed  of  well  seasoned 
white  oak.  The  stiffness  is  provided  by  ^-in.  steel 
tierods.  The  surface  is  covered  by  Monel  metal 
cloth,  24  meshes  to  the  inch,  reinforced  with  No.  10 
gage  galvanized  wire.  Each  screen  revolves  on  a 
bearing  at  the  closed  end  and  on  trunnions  at  the 
open  end,  and  is  driven  by  a  2-hp.  motor. 

The  trickling  filters  cover  an  area  of  30  acres, 
divided  into  10  beds.  They  are  filled  to  an  average 
depth  of  8%  ft.  with  1-  to  2%-in.  hard  broken 
stone.  The  stones  rest  upon  slotted  tile  which  cov¬ 
ers  the  numerous  underdrains  leading  to  a  larger 
underdrain  and  finally  to  the  settling  basins.  These 
beds  are  provided  principally  with  the  Merritt 
square  nozzles,  although  a  few  Taylor  nozzles  are 
still  in  use. 

The  flow  onto  these  beds  is  regulated  from  the 
control  house,  which  contains  a  constant-head 
chamber,  valves  electrically  operated  for  turning 
on  and  off  an  entire  three-acre  bed,  and  butterfly 


4  NINE  YEARS’  OPERATION  OF  THE 

Talves  for  causing  a  fluctuation  in  the  spray,  which 
has  a  cycle  of  approximately  four  minutes. 

The  settling  basins  are  275  x  290  ft.  in  plan  and 
10  ft.  deep,  with  sloping  bottoms. 

The  power  house  is  equipped  with  two  150-hp. 
turbines.  A  direct  connection  to  two  110-kw. 
generators  is  established  by  horizontal  shafts  pass¬ 
ing  through  stuffing  boxes  in  the  wall  of  the  build¬ 
ing.  The  generators  supply  2,300  volts  to  be  stepped 
down  to  220  and  110  volts.  The  power  house  is 
also  equipped  with  a  large  switchboard,  one  two- 
stage  centrifugal  pump  of  600  gal.  per  min.  ca¬ 
pacity  against  a  157-ft.  head,  and  a  gas  engine 
which  was  used  for  driving  one  of  the  generators 
before  the  sewage  flow  was  sufficient  for  operating 
the  turbines  continuously. 

The  sand  beds  are  of  three  distinct  types.  The  orig¬ 
inal  bed  consisted  of  30  in.  of  sand  upon  6  in.  of 
gravel,  but  because  of  poor  drainage  the  sand  depth 
has  been  decreased  from  time  to  time  until  at  pres¬ 
ent  it  is  18  in.  and  drains  satisfactorily.  The  settling- 
basin  sand  beds  have  a  12-in.  depth  of  sand  with  no 
gravel  except  over  the  underdrains.  The  beds  built 
in  1914  aggregate  5.17  acres  and  are  constructed 
with  4  in.  of  sand  on  an  11-in.  layer  of  gravel. 

Normal  Operation  Methods — Baltimore  has  sep¬ 
arate  sewers  connected  to  the  sewage-works  by  a 
single  large  outfall  sewer  30,370  ft.  long,  12  ft.  3  in. 
wide  and  11  ft.  high.  The  sewage  first  passes 
through  the  bar  screens,  depositing  an  average  of 
35  wheelbarrow  loads  of  rags,  fruit  peelings,  sticks, 
etc.,  per  day  or  30  lb.  of  dry  screenings  per  1,000- 
000  gal.  This  material  is  removed  by  hand  and 
sold  to  neighboring  farmers.  The  liquid  then  con¬ 
tinues  on  through  the  venturi  meters,  where  it  is 
divided,  part  gomg  to  the  hydrolytic  and  part  to 
the  Imhoff  tanks.  The  hydrolytic  tanks  are  kept  in 
operation  until  they  show  signs  of  undesirable 
septic  action  or  until  the  laboratory  analyses  show 
the  settling  efficiency  to  be  below  normal.  The  su¬ 
pernatant  liquid  is  pumped  to  one  of  the  other 


BALTIMORE  SEWAGE  WORKS  § 

hydrolytic  tanks  and  the  residual  sludge  is  pumped 
into  any  one  of  the  19  separate  sludge  tanks. 
The  Imhoff  tanks,  on  the  other  hand,  are 
run  continuously  except  for  a  short  interval  every 
two  weeks  when  about  600  cu.ft.  of  sludge  is  run 
out  of  each  tank  producing  good  sludge.  The 
effluents  of  the  hydrolytic  and  Imhoff  tanks  dis¬ 
charge  into  the  same  channel  and  from  there  pass 
through  the  revolving  screens.  Here  the  finer 
particles  which  adhere  to  the  under  side  of  the 
screens  are  washed  off  by  a  continuous  spray  of  pur¬ 
ified  effluent  which  has  been  pumped  from  the  hy¬ 
dro-electric  power  house  to  a  tower  128  ft.  high. 
The  return  water  from  these  screens  is  then  run 
through  two  of  the  Imhoff  tanks  used  temporarily 
for  this  purpose.  A  special  tank  to  care  for  this  re¬ 
turn  water  will  be  constructed.  The  sewage  then 
passes  on  through  the  control  house  to  the  trickling 
filter  beds  and  from  there  through  the  final  settling 
basins  to  the  power  house,  thence  through  the  land 
conduit,  and  is  finally  discharged  into  the  river 
2,500  ft.  from  the  shore  line. 

The  sludge  from  the  hydrolytic  tanks  having  been 
pumped  into  digestion  tanks  is  removed  from  these 
when  in  the  proper  condition,  spread  upon  the 
sludge  drying  beds  to  a  depth  of  12  in.  and,  when 
dry  enough,  is  removed  by  cars  to  the  Heineken 
Reduction  Co.^s  drying  plant,  where  the  material 
is  dried  still  more,  bagged  and  shipped  for  fer¬ 
tilizing  purposes.  The  Imhoff  tank  sludge  is  re- 
m.oved  from  the  drying  beds  by  farmers  who  pur¬ 
chase  it  for  25c  per  load. 

There  being  two  settling  basins,  one  is  cleaned 
while  the  other  operates,  the  cleaning  time  being 
determined  under  conditions  similar  to  those  for 
the  preliminary  sedimentation  tanks.  These  tanks 
are  not  normally  cleaned  as  frequently  as  the  pri¬ 
mary  tanks.  When  neither  is  cleaning  both  are  in 
operation.  The  sludge  from  these  tanks  is  either 
disposed  of  directly  upon  a  sand  bed  provided  for 
this  purpose  or,  when  not  in  the  proper  condition, 


6  NINE  YEARS’  OPERATION  OF  THE 

it  is  pumped  to  any  available  sludge  digestion  tank. 

OPERATION  RESULTS  FOR  NINE  FULL  YEARS 

Actual  Operation  1911  to  1920  Inclusive — Al¬ 
though  the  above  outline  is  the  method  of  opera¬ 
tion  to  which  it  is  well  to  adhere,  actual  conditions 
have  not  allowed  this  procedure  at  all  times.  There¬ 
fore  an  attempt  will  be  made  to  show  just  what  the 
more  important  changes  have  been  and  at  the  same 
time  figures  will  be  presented  to  show  the  results  of 
such  operation. 

On  Nov.  6,  1911,  the  first  quantity  of  sewage 
recorded  as  having  reached  the  plant  was  2,411,000 
gal.,  the  same  being  a  seven  days'  flow.  Prom  this 
time  on  until  February,  1912,  no  consistent  records 
were  kept  but  after  the  latter  date  both  operation 
and  laboratory  analyses  records  were  made  regu¬ 
larly. 

During  these  early  days  the  plain  sedimentation 
tanks  were  used  as  a  dosing  chamber,  the  trickling 
filters  being  operated  but  once  a  day  and  then  only 
for  a  short  interval.  As  more  laterals  were  con¬ 
nected  the  fill-and-draw  method  of  operation  was 
abandoned  and  the  continuous  flow  for  one  bed 
adopted.  The  1912  records  show  an  average  daily 
flow  of  11,800,000  gal.,  whereas  the  1920  flow  was 
52,120,000  gal.  per  day. 

Owing  to  the  fact  that  the  outfall  sewer  had 
been  designed  to  care  for  a  1,000,000  people  at 
150  gal.  per  day  the  first  few  years  of  operation 
with  a  small  flow  saw  the  deposition  of  considerable 
solid  matter  on  the  bottom  of  the  outfall  sewer, 
as  well  as  a  distinctly  septic  sewage  to  be  handled. 
The  former  difficulty  could  only  be  overcome  by 
cleaning  the  lower  end  of  the  outfall  until  such  a 
time  as  the  flow  should  become  large  enough  to 
produce  a  scouring  velocity.  The  cleanings  were 
made  about  twice  yearly,  the  last  in  1916.  At  the 
present  time,  with  a  normal  peak  load  of  69,970, 
000  gal.,  the  deposit  from  the  outlet  end  to  a 
point  two  miles  up  stream  varies  from  4  to  28  in. 


BALTIMORE  SEWAGE  WORKS  7 

The  cleaning  was  accomplished  in  the  following 
manner:  An  adjustable  dam,  in  cross-section  the 
same  shape  as  the  lower  part  of  the  sewer  but  1  ft. 
less  in  diameter,  was  hinged  to  the  rear  of  a  scow 
{See  Engmeering  News,  July  22,  1915,  p.  178).  To 
take  the  scow  up  stream  a  rope  was  attached  to  it 
and  a  man  in  a  rowboat  carried  the  free  end  to 
the  first  manhole,  where  he  tied  it  to  the  iron  steps. 
Then  the  men  in  the  scow  pulled  themselves  up  to 
this  point.  The  same  operation  was  repeated  from 
manhole  to  manhole,  the  dam  floating  in  a  horizon¬ 
tal  position.  When  ready  to  start  cleaning  the  dam 
was  placed  in  a  vertical  position  and  braced  with 
guy  ropes  which  could  be  quickly  released  if  neces¬ 
sary.  The  rapid  flow  of  water  beneath  the  scow 
and  dam  scoured  the  deposit  and  as  the  water 
backed  up  the  dam  would  be  suddenly  released, 
thus  carrying  the  sediment  along.  This  procedure 
was  not  abandoned  because  the  velocity  had  be¬ 
come  sufficient  to  scour,  although  there  is  consid¬ 
erable  improvement  in  this  matter,  but  was  dis¬ 
continued  because  the  volume  of  sewage  is  at  all 
times  of  such  an  amount  as  to  keep  the  flow  line 
too  high  to  allow  sufficient  head  room  for  the  scow 
and  workmen. 

HYDROLYTIC  AND  IMHOFF  TANKS 

Preliminary  Sedimentation  or  Hydrolytic  Tanks — 

During  the  early  years  of  operation  the  small  vol¬ 
ume  of  sewage  received  at  the  plant  made  it  pos¬ 
sible  to  clean  the  hydrolytic  tanks  every  ten  days  to 
three  weeks  in  the  summer,  and  six  weeks  in  the 
winter.  When  cleaned  this  often  very  little  water 
had  to  be  added  to  cause  the  sludge  to  run  to  the 
pump  well  from  which  it  was  delivered  to  the 
separate  digestion  tanks.  This  condition  held  be¬ 
cause  the  scum  had  not  had  sufficient  time  to  be¬ 
come  thick  and  dry.  At  this  time  there  was  always 
one  hydrolytic  tank  in  service,  one  being  cleaned 
and  one  in  reserve.  In  June,  1916,  the  flow  had 
become  too  great  for  one  tank  and  a  second  one 


8  NINE  YEARS’  OPERATION  OF  THE 

had  to  be  placed  in  service  for  the  peak  load, 
making  two  in  parallel.  Although  the  Imhoff  tanks 
began  operating  in  August,  1915,  it  was  neces¬ 
sary  to  continue  the  operation  of  two  hydrolytic 
tanks,  and  eventually  to  run  the  two  continuously 
in  parallel.  The  reason  for  this  will  be  mentioned 
when  the  Imhoff  tank  operation  is  considered.  A 
gradual  accumulation  of  sludge  has  put  to  use  all 
the  available  storage  capacity  provided  by  the  sep¬ 
arate  digestion  tanks.  This  has  resulted  in  less 
frequent  cleaning  of  the  hydrolytic  tanks,  which  in 
turn  has  produced  a  heavy  scum,  pretty  well  dried 
out.  This  has  of  necessity  increased  the  cost  of 
cleaning  because  considerable  water  must  be  added 
to  remove  the  scum  and  sludge  to  the  digestion 
tanks. 

Soundings  made  from  1915  through  1920  show 
that  the  cubic  yards  of  90  per  cent  undigested 
sludge  produced  per  million  gallons  of  sewage  treat¬ 
ed  has  been  5.70,  5.53,  7.34,  6.06,  5.14  and  6.37  for 
the  respective  years  during  this  period.  An  average 
detention  period  of  3^  hr.  produces  the  best  efflu¬ 
ent,  and  approximately  65  per  cent  efficiency,  based 
upon  settling  solids  removed,  can  be  maintained 
under  normal  conditions. 

Imhoff  Tanks — The  operation  of  the  Imhoff  tanks 
began  in  August,  1915,  when  7  of  ’ the  28  were  put 
into  service.  Each  tank  was  ‘‘seeded^’  differently 
to  determine  the  best  method  of  producing  well-di¬ 
gested  sludge  in  the  minimum  length  of  time. 
Among  some  of  the  methods  used  was  the  addition 
of  newspapers,  lime,  digested  sludge,  etc.,  but  no 
advantage  was  found  over  the  usual  method  of 
allowing  the  sludge  to  accumulate  and  digest  in 
the  natural  way.  Other  tanks  were  gradually  put 
into  service  until  by  March,  1916,  all  but  two, 
which  were  reserved  for  activated-sludge  experi¬ 
ments,  were  operating. 

The  tanks  were  operated  at  a  low  rate  during  the 
ripening  period,  which  was  rather  long,  but  as  soon 


BALTIMORE  SEWAGE  WORKS  9 

as  proper  bacterial  action  seemed  to  have  set  in 
the  rate  was  increased.  Considerable  trouble  was 
soon  experienced  with  foaming  and  excessive  scum 
formation.  Sludge  was  removed  as  frequently  as 
conditions  would  warrant,  there  being  times  when 
nothing  but  water  could  be  drawn  off,  the  sludge 
having  gone  to  scum.  The  sloping  walls  were 
squeegeed  by  a  regular  attendant,  but  in  spite  of  all 
this  the  scum  in  the  sedimentation  chamber  at  times 
reached  a  thickness  of  2  ft.  and  more. 

To  prevent  and  quiet  foaming  a  fire  hose  was 
used,  but  the  best  method  was  to  shut  the  tank  off 
from  a  few  days  to  two  or  three  weeks,  during 
which  interval  skimming  of  the  sedimentation 
chamber  and  spraying  of  the  gas  stack  were  usually 
resorted  to.  The  tanks  were  then  restarted  very 
slowly. 

Although  these  tanks  were  designed  to  treat 
500,000  gallons  each,  per  day,  with  a  50  per  cent 
overload,  the  average  maintained  for  about  two 
years  for  26  tanks  was  4,800,000  gal.  per  day. 
That  is  to  say,  no  more  than  this  amount  could  be 
treated  with  a  proper  settling  efficiency  and  with¬ 
out  considerable  scum  escaping.  At  this  rate,  how¬ 
ever,  the  average  efficiency  maintained  was  61.6 
per  cent. 

'MHOFF  TANKS  TAKEN  OUT  OF  SERVICE 

On  June  6,  1917,  the  Imhoff  system  was  taken  out 
of  service  because,  at  this  time,  the  volume  of  flow 
had  become  too  great  for  one  sedimentation  tank 
and  the  Imhoff  tanks  with  their  limited  capacity, 
and  not  enough  for  two  hydrolytic  tanks  and  the 
Imhoffs.  That  is,  if  the  latter  combination  were 
used  the  detention  period  in  the  hydrolytic  tanks 
would  become  so  long  that  odors  would  be  materially 
increased.  Another  reason  for  discontinuing  the 
Imhoff  tanks  was  one  purely  local  and  strictly  of  a 
hydraulic  nature.  Because  of  the  excessive  loss  of 
head  through  the  Imhoff  system  the  water  level  in 
the  large  effluent  channel  leading  to  the  revolving 


10  NINE  YEARS’  OPERATION  OF  THE 

screens  had  to  be  carried  a  foot  lower  than  when 
the  hydrolytic  tanks  operated  by  themselves,  there¬ 
by  reducing  the  available  area  of  the  revolving 
screens  very  considerably.  Furthermore,  as  soon 
as  the  head  in  the  outfall  sewer  dropped  below  a 
certain  elevation  the  Imhoff  system  would  stop  op¬ 
erating  and  then  as  the  flow  increased  the  water 
would  seek  its  level  through  the  hydrolytic  tanks 
because  of  the  less  loss  of  head  through  them.  In 
consequence  of  this  the  sewage  would  enter  the 
Imhoff  tanks  backward,  thereby  disturbing  the 
scum,  pieces  of  which  would  leave  the  tanks  for 
an  hour  or  more  after  they  had  started  operating. 
This  condition  also  tended  to  overwork  the  revolv¬ 
ing  screens. 

When  the  Imhoff  system  was  discontinued  the 
average  daily  rate  of  flow  was  44,380,000  gal.,  but 
by  1919  this  had  increased  to  55,060,000  gal.  This 
made  it  possible  to  restart  the  Imhoff  system  by 
slightly  increasing  the  detention  period  in  the  hy¬ 
drolytic  tanks.  With  this  end  in  view  two  tanks 
were  remodeled  and  put  in  operation  in  May,  1919, 
for  experimental  purposes.  The  results  of  a  long 
period  of  experiment  warranted  a  change  being 
made  in  all  the  tanks.  Accordingly  this  work  was 
carried  on  as  rapidly  as  possible  and  by  January, 
1921,  18  of  the  28  tanks  were  in  service.  The  al¬ 
terations  consist  of  a  raise  in  the  flow  line  so  that 
there  can  be  no  back  flow  into  the  tanks,  reducing 
the  number  of  weirs  in  the  outlet  channel  of  each 
tank  from  52  to  35,  reducing  their  depth 
to  1  in.  and  closing  permanently  all  but 
one  of  the  ports  in  the  bottom  of  the  influent  ring 
around  the  gas  stack.  This  one  port  is  closed  with 
a  wooden  plug  to  which  is  attached  a  long  handle 
so  that  the  same  can  be  withdrawn  once  or  twice 
a  day  to  flush  the  sediment  out  of  the  influent 
channel.  The  changes  as  made  cause  the  tanks  to 
operate  with  a  radial  horizontal  flow  instead  of  a 
radial  downward  and  outward  flow. 

The  tanks  now  in  operation  give  results  prac- 


11 


BALTIMORE  SEWAGE  WORKS 

tically  as  good  as  the  two  run  experimentally, 
showing  an  average  efficiency  of  72.8  per  cent, 
with  a  rate  of  365,000  gal.  per  tank  per  day  during 
1920,  as  against  61.6  per  cent,  with  a  rate  of  285,- 
000  gal.  under  the  designed  method  of  operation. 


FIG.  2.  DAILY  RATES  OP  FLOW  AND  UNIT  COST  OP 
TREATMENT,  BALTIMORE  SEWAGE-WORKS 

Although  they  are  not  up  to  the  designed  ca¬ 
pacity  of  500,000  gal.  per  tank  per  day,  it  is  the 
writer’s  belief  that  approximately  this  quantity  could 
be  treated  if  the  hydraulic  features  were  such  that 
the  tanks  could  operate  the  full  24  hrs.  instead  of  20. 
It  is  interesting  to  note  that  with  the  revised  tanks 
little  scum  has  formed  in  the  sedimentation  cham¬ 
ber  except  for  a  short  interval  when  the  scum  in  the 
gas  stack  became  so  thick  that  it  apparently  short 
circuited  and  came  up  through  the  slot  at  the 
base  of  the  flare  wall.  By  removing  some  scum 


12  NINE  YEARS’  OPERATION  OF  THE 

from  the  gas  stack,  thereby  raising  the  bottom 
of  the  scum  layer  above  the  slot,  this  trouble  was 
overcome.  Very  little  trouble  was  caused  by  foam¬ 
ing,  though  when  it  did  occur,  it  was  easily  con¬ 
trolled  by  the  use  of  a  hose  for  a  few  moments. 

Revolving  Screens — In  1912  two  large  revolving 
screens  constructed  of  cast  iron  and  steel,  weighing 
approximately  ten  tons  each  and  costing  $9,645, 
were  put  in  service.  Because  of  their  excessive 
weight  they  wore  out  the  trunnions  in  ten  days 
or  less,  whereas  repairing  the  damage  took  about 
two  weeks  per  screen.  Thus  they  gave  actual  ser¬ 
vice  less  than  half  the  time  besides  being  very 
costly.  After  fair  trial  they  were  abandoned  and 
the  wooden  screens  designed  by  J.  J.  Neal,  form¬ 
erly  superintendent  of  the  plant,  superseded  them. 
As  stated  before,  they  were  constructed  prin¬ 
cipally  of  oak,  were  built  with  the  plant  force  and 
weigh  about  1^  tons  each.  There  are  four  of 
these,  the  first  of  which  was  installed  in  January, 
1914.  The  others  have  been  built  as  needed  and 
in  consequence  have  cost  different  sums  which  may 
be  stated  as  approximately  $1,100,  $1,500,  $1,800 
and  $2,500,  or  a  total  of  $6,900.  They  have  given 
excellent  satisfaction,  no  one  screen  having  been 
out  of  service,  on  an  average,  more  than  two  weeks 
per  year  because  of  mechanical  difficulties.  The 
Monel  metal  cloth  is  renewed  from  time  to  time 
and  the  screens  are  rewound  with  reinforcing  wire. 
All  told,  each  screen  operates  about  90  per  cent  of 
the  time.  The  only  important  change  in  construc¬ 
tion  is  the  substitution  of  a  24-mesh  screen  for  the 
26-mesh  since  the  latter  has  given  trouble  by  clog¬ 
ging. 

An  idea  of  the  efficiency  of  the  screens  is  obtained 
by  noting  that  when  in  service  from  5  to  15  pei 
cent  of  the  sprinkling  nozzles  are  cleaned  daily  as 
against  37  to  141  per  cent  when  not  running.  As 
a  result  two  men  working  eight  hours  daily  are 
able  to  keep  30  acres  of  sprinkling  filters  looking 
well  practically  all  the  time.  Furthermore,  a  series 


BALTIMORE  SEWAGE  WORKS  12 

©f  analyses  made  in  1919  show  that  the  return 
water  from  these  screens  contains  186  p.  p.  m.  of 
susp>ended  solids,  consisting  principally  of  sur¬ 
face  floating  material,  such  as  grease,  lumps  of 
scum,  etc.  Approximately  100,000  gal.  of  purified  sew¬ 
age  per  screen  is  used  daily  for  washing  purposes. 

TREATED  EFFLUENT  USED  TO  WASH 

SCREENS 

The  purified  effluent  used  as  a  spray  on  the  re¬ 
volving  screens  is  conveyed  to  them  by  means  of 
2^/^ -in.  pipes  placed  horizontally  over  the  upper¬ 
most  part  of  the  screens.  As  the  screens  revolve 
with  the  suspended  matter  adhering  to  the  under 
side  of  the  wire  mesh  the  spray  of  purified  effluent 
washes  this  material  off  into  a  trough  beneath  the 
spray,  which  returns  it  to  the  Imhoff  tanks  pre¬ 
viously  referred  to.  The  pipes  were  formerly 
equipped  with  a  number  of  nipples  having  3-16-in. 
orifices  and  acted  as  a  reciprocating  arm  by  means 
of  a  cam  attachment.  The  orifices  clogged  readily 
and  were  hard  to  clean  and  the  reciprocating  arm 
gave  mechanical  troubles.  Because  of  these  facts 
the  arrangement  described  was  replaced  by  sta¬ 
tionary  pipes  with  an  0.008-in.  width  slot  cut  on 
the  under  side.  Any  clogging  is  now  easily  removed 
by  sliding  back  and  forth  through  the  pipe  and  on 
the  under  side  of  the  slot  a  small  V-shaped  piece  of 
metal  attached  to  the  end  of  a  long  pole.  The  ma¬ 
terial  broken  loose  in  this  manner  is  then  removed 
from  the  pipe  by  flushing. 

The  Trickling  Filter  Control  House — This  has  op¬ 
erated  continuously,  as  designed,  except  for  one 
or  two  changes.  The  automatic  float  switches 
for  operating  the  filter  beds  have  been  abandoned 
because  the  operator  was  not  always  present  to 
record  the  changes  as  they  took  place.  Now  the 
switches  are  operated  by  hand  whenever  the  water 
level  in  the  channel  leading  to  the  control  house 
rises  above  or  falls  below  a  given  elevation.  This 
arrangement  also  allows  a  better  operation  of  the 


14  NINE  YEARS’  OPERATION  OF  THE 

Imhoff  and  hydrolytic  tanks  in  parallel.  The  loss 
of  head  through  the  operating  gates  leading  to  the 
control  house  constant-head  chamber  proved  to 
be  so  great  that  one  of  these  gates  had  to  be 
opened  permanently.  The  butterfly  valves,  driven 
by  friction  disks,  have  given  excellent  satisfaction 
although  the  blades  have  been  broken  or  washed 
away  occasionally,  which  has  resulted  in  minor, 
changes  of  reinforcement. 

The  Trickling  Filters — In  1912  the  Alter  beds 
operated  30.69  acre-days  per  week  as  against  125.08 
acre-days  per  week  at  present.  As  additional 
beds  were  required  a  new  bed  was  ripened  by 
using  it  for  a  short  interval  each  day  at  the  start, 
then  gradually  increasing  the  daily  period  until, 
after  about  six  weeks,  it  would  be  found  normal. 
After  being  ripened  the  beds  would  be  so  rotated 
that  the  new  one  ordinarily  would  operate  at 
least  one  hour  daily  and  was  never  idle  more  than 
three  days  at  a  time.  They  have  sluffed  regularly 
and  apparently  to  such  an  extent  that  from  all 
indications  it  will  not  be  necessary  to  remove  the 
filter  stone  for  cleaning  at  the  end  of  ten  years, 
as  predicted  by  the  advisory  engineers,  and  it  is 
doubtful  that  they  will  ever  need  to  be  removed 
for  cleaning  purposes.  The  pipe  line  installed  in 
the  galleries,  carrying  purified  sewage  for  flushing 
the  underdrains,  has  not  been  used  for  this  pur¬ 
pose.  However,  the  lateral  distributing  pipes  have 
to  be  flushed  occasionally.  This  was  originally  done 
by  opening  the  valves  in  the  galleries  at  the  end 
of  each  distributor  and  allowing  the  sediment  to 
drain  into  a  wooden  trough  which  emptied  into 
the  filter  channel  leading  to  the  settling  basins. 
This  method  was  not  entirely  satisfactory  because 
of  the  splashing  over  the  trough  and  into  the 
small  underdrains.  Now  the  laterals  are  flushed 
by  removing  the  nozzle  at  the  end  of  each  distribut¬ 
ing  line,  leaving  it  off  during  two  cycles  of  the 
butterfly  valve  and  then  replacing  the  nozzle.  Thus 
all  sewage  passes  through  the  filter  beds.  Occasion- 


BALTIMORE  SEWAGE  WORKS 


15 


ally  it  is  necessary  to  flush  out  this  section  of  each 
bed  with  a  hose.  The  laterals  on  each  bed  are 
flushed  about  once  every  ten  days. 

FILTER  OPERATION 

The  cleaning  of  nozzles  referred  to  when  speaking 
of  the  revolving  screens  is  accomplished  by  remov¬ 
ing  the  nozzles  and  knocking  the  sediment  off  while 
the  beds  are  running  at  the  cleaning  point,  or  at  ap¬ 
proximately  one-third  the  quantity  of  regular  ser¬ 
vice.  At  times  of  excessive  flow,  the  butterfly  valves 
are  stopped  entirely,  so  that  the  flow  through  the 
beds  is  constant  and  at  its  maximum  rate,  this  being 
1.39  times  the  normal  rate.  Although  the  advisory 
engineers  recommended  a  rate  of  2,500,000  gal.  per 
acre  per  day  the  rate  has  varied  considerably  at 
times,  but  the  yearly  averages  shown  in  Table  I  have 
been  maintained  with  generally  good  nitrification. 

TABLE  I.  AVERAGE  YEARLY  OPERATION  DATA 


BALTIMORE  TRICKLING  FILTERS. 

Rate  of  Filtration  Nitrates  Relative  Stability 


Year 

M.  G.  per  Acre  Daily 

P.  P.  M. 

Number 

1912 

2.73 

8.3 

93 

1913 

2.64 

8.2 

91 

1914 

2.72 

7.2 

78 

1915 

2.88 

6.2 

87 

1916 

2.84 

4.4 

93 

1917 

2.96 

7.4 

92 

1918 

3.00 

8.1 

96 

1919 

2.79 

7.7 

89 

1920 

2.91 

7.0 

92 

In 

spite  of  the  regular  cleaning  of  the  nozzles 

thin  scale  forms  on  the  under  side  of  the  deflector 
plate,  altering  the  radius  of  the  spray  to  such  an 
extent  that  the  scale  must  be  removed.  This  has 
been  accomplished  in  two  ways,  one  of  which  can  be 
done  in  warm  or  windy  weather  only.  That  is,  the 
bed  is  allowed  to  be  idle  for  about  two  days,  when 
this  scale  becomes  thoroughly  dried.  The  flow  is 
then  turned  on  and  the  scale  flushes  off.  This  method 
is  objectionable  because  of  the  strong  odor  arising 
from  the  bed  treated  in  this  manner.  The  other 
method  is  efficient  but  laborious.  The  nozzle  is 
bciled  in  lye  water,  scrubbed  with  a  steel  brush  and 
then  replaced. 


16  NINE  YEARS’OPERATION  OP  THE 

Two  alterations  have  been  made  in  the  nozzles 
proper.  The  threads  of  the  cast-iron  nozzle  plates 
had  gradually  deteriorated  until  the  nozzle  would 
no  longer  hold  in  place. 

Instead  of  using  new  plates  a  threaded  brass  bush¬ 
ing  has  been  placed  in  the  plates  needing  this  re¬ 
vision  and  their  life  seems  to  be  longer  than  under 
the  original  condition.  Approximately  4,000  of  the 
5,830  nozzle  plates  have  been  altered  in  this  manner. 
The  other  change  has  been  to  cut  the  threads  off  of 
the  deflector  plate  stem  as  they  became  worn  and 
loose  and  to  rivet  the  stem  into  the  nozzle  base. 

TWO  BASINS  RUN  IN  PARALLEL 

The  Final  Settling  Basins — These  basins,  of  which 
there  are  two,  were  recommended  for  a  3-hr.  deten¬ 
tion  period,  whereas  actually  the  average  for  all  the 
years  of  operation  has  been  6.04  hrs.  Originally  it 
was  possible  to  operate  one  basin  while  the  other 
was  being  cleaned  or  held  in  reserve.  As  the  flow 
grew  larger  it  became  necessary  to  run  both  basins 
in  parallel.  Then  the  loss  of  head  through  the  mix¬ 
ing  chamber  and  inlet  valves  to  the  settling  basins 
proved  to  be  so  great  that  it  was  necessary  to  raise 
the  walls  of  the  channel  leading  to  the  mixing  cham¬ 
ber,  to  build  a  bypass  around  it  (to  the  settling 
basins)  and  to  make  additional  inlet  openings  into 
the  basins.  It  might  be  said,  in  passing,  that  the 
mixing  chamber,  originally  installed  for  the  addition 
of  hypochlorite  of  lime,  has  never  been  used  except 
experimentally,  and  then  only  once. 

The  Hydro-Electric  Power  House — This  part  of 
the  plant  has  been  operated  as  designed  except  for 
minor  mechanical  and  electrical  changes.  When  the 
plant  first  started  it  was  necessary  to  use  the  gaso¬ 
line  engine  for  generating  electricity  part  of  each 
day,  but  there  has  been  sufficient  sewage  at  all  times 
since  early  in  1916  to  allow  the  permanent  closing 
down  of  this  engine. 


BALTIMORE  SEWAGE  WORKS  17 

SLUDGE  DISPOSAL 

Here  at  Baltimore,  as  well  as  at  practically  all 
disposal  works,  sludge  disposal  is  of  primary  im¬ 
portance.  As  first  designed  three  square  digestion 
tanks  of  609,000  cu.  ft.  capacity  were  provided.  In 
1914  sixteen  additional  tanks  of  the  circular  type, 
with  a  volume  of  330,000  cu.  ft.,,  were  built.  A  con¬ 
siderable  addition  to  the  sand  bed  area  was  likewise 
provided.  However,  the  sludge  has  continued  to  ac¬ 
cumulate  at  a  faster  rate  than  it  has  been  disposed 
of,  so  that  an  addition  to  the  digestion  tanks  is  now 
under  construction,  while  more  beds  will  be  provided 
in  the  near  future. 

Although  troublesome  at  times,  our  sludge  oper¬ 
ation  experience  has  been  interesting.  In  the  first 
years  of  plant  operation  there  was  plenty  of  storage 
capacity  so  that  sufficient  time  could  be  allowed  to 
produce  well-digested  sludge  for  disposal  upon  the 
sand  beds.  This  sludge  drained  readily,  dried  and 
cracked  well.  During  digestion  the  scum  on  the 
tanks  formed  from  a  few  inches  to  approximately  2 
ft.  in  thickness,  but  at  no  time  prevented  the  nor¬ 
mal  operation  of  these  tanks,  which  was  as  follows: 
The  undigested  sludge  was  pumped  from  the  hy¬ 
drolytic  tanks  into  the  digestion  tanks,  being  de¬ 
livered  at  one  side  and  just  beneath  the  surface. 
The  digestion  tank  receiving  the  sludge  would  then 
lie  undisturbed  over  night,  during  which  period  the 
water,  which  had  been  added  to  the  undigested 
sludge  during  the  pumping,  would  separate  from 
the  sludge  and  rise  to  a  place  just  beneath  the 
scum.  From  here  the  water  was  drawn  off  by 
means  of  a  channel  near  the  top  of  these  digestion 
tanks  and  was  disposed  of  in  the  idle  hydrolytic 
tank  or  the  one  which  had  been  in  service  longest. 
This  tank  would  then  be  ready  to  receive  more 
undigested  sludge  or  for  the  removal  of  some  of 
the  digested.  The  digested  sludge  was  removed 
by  opening  one  of  the  three  well  gates  on  the 
side  opposite  the  incoming  sludge.  The  sludge 


18  NINE  YEARS’  OPERATION  OF  THE 

was  drawn  from  the  sloping  bottom  through  any 
one  of  the  three  drain  channels  to  a  collecting 
well  and  from  there  through  a  drain  to  the  pump 
well,  from  which  place  it  was  delivered  to  the 
sand  beds. 

It  was  noticed  that  certain  parts  of  these  tanks 
remained  dead  because  there  were  only  three  draw¬ 
off  points.  To  remedy  this  two  of  the  tanks  were 
emptied  entirely  and  the  drain  channels  covered  with 
concrete  slabs.  Four  lift  gates  were  placed  across 
the  tank  on  each  channel,  making  12  in  all  for  each 
tank.  The  idea  was  to  raise  by  a  chain  that  gate 
serving  the  part  of  the  tank  from  which  it  was  de¬ 
sired  to  draw  the  sludge.  After  the  tanks  were  full 
and  in  operation  unfortunately  the  gates  could  not 
be  lifted  by  less  than  two  men  and,  when  once 
raised,  it  is  doubtful  whether  they  closed  again. 

In  1914  the  scum  had  started  to  form  from  2  to 
2^  ft.  in  thickness  and  a  hose  was  played  upon 
the  surface  to  break  it  up.  This  was  unsuccessful 
as  the  area  was  so  large  that  the  force  of  the  water 
was  spent  by  the  time  it  reached  the  center  of  the 
tanks.  Sludge  was  continuing  to  form  at  an  in¬ 
creasing  rate  and  weather  conditions  prevented 
rapid  drying  on  the  limited  sand  bed  area,  so  that 
when  the  new  circular  tanks  and  sand  beds  were 
completed  in  1915  they  were  immediately  pressed 
into  service.  In  spite  of  this  the  sludge  continued 
to  accumulate  and  by  the  later  part  of  1916  there 
was  again  need  of  more  storage  capacity.  This 
seemed  preferable  to  more  sand  bed  area  since  the 
weather  conditions  regulate  to  a  great  extent  the 
rate  at  which  the  beds  can  be  cleaned  and  because 
the  area  then  in  use  was  about  sufficient  for  the 
available  laboring  force  even  when  the  sludge  was 
drying  satisfactorily.  Table  II  gives  an  idea  of  the 
sludge  situation  as  it  shows  the  total  sewage  flow 
for  each  year,  the  digested  sludge  produced  and  dis¬ 
posed  of  and  the  quantity  in  the  tanks  at  the  end  of 
each  year.  The  column  of  sludge  disposed  of  gives 


BALTIMORE  SEWAGE  WORKS  19 

the  total,  which  includes  that  placed  on  sand  beds, 
sold  to  farmers  and  placed  on  other  areas. 

TABLE  II.  DIGESTED  SLUDGE  PRODUCTION 
AND  DISPOSAL  AT  BALTIMORE  SEWAGE-WORKS 

Cubic  yards  of  90  per  cent  Moisture 
Digested  Sludge  Sewage 

Flow  Produced  Pro¬ 


M.  G. 

per  M. 

G.  duced 

Disposed 

In  Tanks 

per 

(Based 

on  per 

of  per 

At  End  of 

Year 

Year 

Soundings)  Year 

Year 

Year 

1912 

4,320.85 

1.52 

6,568 

No  record 

No  record 

1913 

6,597.48 

1.72 

11,348 

No  record 

No  record 

1914 

8,372.09 

3.85 

32,233 

24,981 

15,168 

1915 

11,607.41 

4.05 

47,010 

49,856 

12,322 

1916 

14.374.00 

3.69 

53,040 

45,760 

19,602 

1917 

16,199.83 

3.15 

51,029 

29,616 

41,015 

1918 

19,284.78 

3.32 

64,025 

39,103 

65,937 

1919 

20,096.08 

2.98 

59,886 

65,617 

60,206 

1920 

19,074.23 

3.00 

57,223 

57,214 

60,215 

More  sludge 

was  produced  during  the  years  1916 

to  1918  than  was  disposed  of,  and  in  consequence 
the  quantity  remaining  in  the  tanks  at  the  end  of 
each  year  grew  larger.  The  ultimate  result  has 
been  a  gradual  change  in  the  moisture  content  of 
the  sludge.  As  previously  stated,  at  first  well-digest¬ 
ed  sludge  was  sufficiently  watery  to  run  freely  from 
the  tanks,  while  the  scum  formation  varied  from 
a  few  inches  to  approximately  2  ft.  By  1914  the 
scum  had  reached  2  to  2V2  ft.  in  thickness  but  it 
was  still  possible  to  remove  well-digested  sludge 
without  the  addition  of  water.  By  1917  the  surface 
scum,  iy2  ft.  thick,  had  dried  to  a  moisture  con¬ 
tent  of  69.9  per  cent.,  with  the  next  2%  ft.  below 
containing  75.9  per  cent  water  and  the  remaining 
depth  sufficiently  watery  to  run.  However,  the  best 
digested  sludge  seemed  to  be  in  these  top  4  ft.,  so 
that  in  order  to  place  the  best  digested  sludge  upon 
the  sand  beds  a  2-inch  hose  working  under  a  128-ft. 
head  was  used  to  break  the  surface  up  and  mix  it 
with  the  more  liquid  sludge.  This  gradual  loss  of  wa¬ 
ter  continued  until  the  worst  conditions  for  the  large 
square  digestion  tanks  was  reached  in  October  1918, 
whereas  the  circular  tanks  present  their  most  diffi¬ 
cult  operation  conditions  at  the  present  time.  In 
fact  the  scum  in  some  of  the  circular  tanks  is  as 


20  NINE  YEARS’  OPERATION  OF  THE 

much  as  11  ft.  thick.  In  October,  1918,  soundings 
and  analyses  showed  the  moisture  content  of  the 
♦op  2  ft  of  the  square  digestion  tanks  to  be  62.4 
per  cent,  while  the  balance  of  the  depth,  or  11^ 
ft.,  except  for  small  pockets  of  water,  contained  78 
per  cent,  of  water. 

To  relieve  these  conditions  various  available  areas 
have  been  utilized  by  filling  with  sludge  cut  up  with 
water.  It  was  felt  that  more  rapid  progress  could 
be  made  by  removing  the  sludge  without  the  ad¬ 
dition  of  water.  With  this  end  in  view  a  paddle 
conveyor,  electrically  driven,  was  put  in  service  in 
April,  1918.  For  part  of  the  time  this  removed  the 
heavy  sludge  to  the  area  adjacent  to  the  square 
digestion  tanks  from  which  place  it  was  hauled 
away  by  the  farmers,  and  for  the  balance  of  the 
time  placed  it  directly  into  cars  for  hauling  to  the 
rotary  drier  plant. 

In  1919  more  sludge  was  disposed  of  than  pro¬ 
duced  when  the  area  between  the  filter  beds 
and  hydrolytic  tanks  was  filled,  thereby  not  only 
helping  the  sludge  situation  but  improving  the  ap¬ 
pearance  of  the  grounds.  The  effect  of  this  was 
noted  in  the  soundings  and  analyses  made  in  the 
large  digestion  tanks  in  October,  1919,  when  the 
scum  moisture  had  increased  from  62.4  to  69.8  per 
cent  and  the  sludge  beneath  had  increased  from 
78  to  79.9  per  cent.  During  1920,  when  the  same 
amount  of  sludge  was  disposed  of  as  produced,  the 
moisture  content  of  the  scum  and  of  the  sludge  be¬ 
neath  was  further  increased,  so  that  on  Oct.  1,  1920, 
the  figures  were  72.4  and  81.4  per  cent  respective¬ 
ly. 

Since  early  in  1918  one  of  the  most  annoying  fea¬ 
tures  of  the  digestion  tank  operation  has  been 
the  appearance  of  undigested  sludge  at  the  outlet 
end  of  the  tanks.  This  has  been  accounted  for  by 
the  fact  that  the  raw  sludge,  as  pumped  into  the 
digestion  tanks  at  one  side,  found  its  way  under  the 
scum  or  through  the  heavy  sludge,  much  as  water 


BALTIMORE  SEWAGE  WORKS 


21 


does  through  a  clay  bank.  However,  the  well-di¬ 
gested  scum,  when  broken  up  and  mixed  with  un¬ 
digested  sludge,  has  enabled  the  placing  of  at 
least  partly  digested  material  upon  the  sand  beds  at 
all  times.  For  this  purpose  a  3^/^ -in.  fire-hose  has 
been  substituted  for  the  former  2-in.  one  and  i» 
proving  very  satisfactory. 

The  operating  force  feels  that  when  the  excess 
sludge  has  been  disposed  of  and  sufficient  storage 
capacity  and  sand  beds  have  been  provided  for  the 
current  accumulation,  so  that  the  tanks  can  be 
kept  with  a  scum  not  more  than  2  ft.  thick  and  the 
sludge  with  not  less  than  89  per  cent  water,  the 
results  will  be  very  satisfactory,  because  past  ex¬ 
perience  has  shown  that  sludge  digests  well  in  the 
separate  tanks. 

One  of  the  interesting  features  of  the  well-di¬ 
gested  sludge  is  that  it  now  contains  57  per  cent 
volatile  matter  as  compared  to  44.2  per  cent  in  1914, 
when  the  tanks  were  operating  normally  and  well. 
This  change  is  probably  due  in  part  to  the  fact  that 
in  the  early  days  of  operation  considerable  mineral 
matter  entered  the  sewers,  due  to  the  constructio* 
work  then  in  progress. 

TABLE  III.  AVERAGE  ANALYSES  OF  SLUDGE 
FROM  DIFFERENT  TANKS  AT  BALTIMORE 

Per  Cent  of  Dry  Residue 

Total 

Phos- 

— Wet  Sludge —  phoric 


Per 

Specific 

Ni-  Acid  Potash 

Cent 

Grav- 

Vola- 

tro- 

as 

as 

Digestion 

Water 

ity 

tile 

Fats 

gen 

PiOs 

K2O 

tank  sludge. 
Imhoff 

.  91.86 

1.021 

66.21 

4.02 

2.45 

0.52 

O.IT 

tank  sludge. 

.  92.38 

1.017 

62.74 

•  •  •  • 

2.75 

0.58 

O.ly 

Raw  sludge* 
Settling 

.  79.16 

73.84 

9.00 

2.64 

•  •  •  • 

•  •  •  • 

basin  sludge 

.  92.37 

57.98 

•  •  •  • 

3.19 

•  •  •  • 

•  •  *  • 

♦Analysis 

as  sludge  exists  in 

tanks 

before 

the 

addi- 

tion  of  water  for  cleaning.  As  the  raw  sludge  is  being 
pumped  into  the  digestion  tanks  the  moisture  content 
is  91.56  per  cent,  and  the  specific  gravity  1.020. 

The  sludge  as  produced  at  this  plant  must  be  con¬ 
sidered  under  four  headings;  namely,  raw,  separate 


22  NINE  YEARS’  OPERATION  OF  THE 

digestion  tank,  Imhoff  tank,  and  final  settling  basin 
sludge.  An  average  analysis  of  these  types  is  pre¬ 
sented  in  Table  III. 

Sand  Beds — The  sludge  is  run  onto  the  sand  beds 
to  a  depth  of  12  in.  and  when  well  digested  cracks 
and  dries  rapidly  in  the  summer,  provided  it  is 
not  an  abnormally  wet  season.  Ordinarily  the 
beds  drain  well,  but  when  one  works  to  the  con¬ 
trary  either  of  two  methods  is  used  to  relieve  the 
water.  A  man  walks  through  the  bed  stirring 
up  the  entire  mass  or  a  small  pipe  is  inserted  at 
one  or  more  corners  of  the  bed  to  drain  the  water 
to  the  adjacent  area.  Then  when  the  bed  is  empty 
the  sand  is  loosened  by  raking. 

In  the  years  1915  to  1920  inclusive  91,369  cu.  yd. 
of  dried  sludge  were  taken  from  the  beds  and  the 
beds  were  cleaned  and  refilled,  on  an  average  of 
S%  times  a  year.  Calculating  from  the  average  of 
3.326  cu.  yd.  per  1,000,000  gal.  of  90  per  cent  di¬ 
gested  sludge  produced  and  30,615  cu.  yd.  of  90% 
sludge  handled  on  the  digestion-tanks  sand  beds 
yearly  they  have  cared  for  the  production  of  sludge 
from  9,205,000,000  gal.  of  sewage.  At  100  gal.  per 
capita  daily,  which  is  approximately  our  average, 
a  yearly,  average  of  252,192  people  have  been  served 
by  5.06  acres  of  sand  bed  area  during  this  five  year 
period.  In  other  words,  each  person  has  required 
0.874  sq.  ft.  of  sand  bed  area  per  year. 

Table  III  shows  that  the  sludge  from  the  digestion 
tanks  as  placed  on  the  sand  beds  contains  an  aver¬ 
age  of  91.86  per  cent  water.  The  range  of  moist¬ 
ure,  however,  is  from  86.6  to  96.4  per  cent.  That 
run  from  the  Imhoff  tanks  ranges  from  85.7  to  94.8 
per  cent  moisture.  When  removed  from  the  beds, 
the  water  content  of  the  sludge  ranges  from  48.6 
to  78.3  per  cent,  with  an  average  of  68.7  per  cent. 
In  removing  the  sludge  it  is  shoveled  into  cars  and 
dumped  in  front  of  the  Heineken  Reduction  Co’s 
drying  plant.  From  the  beginning  of  operation  to 
April,  1916,  this  material  was  placed  in  piles  for 


23 


BALTIMORE  SEWAGE  WORKS 

future  use  in  leveling  off  the  grounds  or  for  sale  to 
the  farmers.  But  on  Feb.  1,  1916,  a  five-year  con¬ 
tract  was  made  for  all  sand-bed  dried  sludge  so 
that  since  then  the  sludge  has  been  dumped  on  this 
company's  storage  pile.  The  cars  were  formerly 
filled  level  full,  removed  from  the  beds  by  hand  and 
then  drawn  by  a  mule  to  the  dumping  ground.  The 
first  part  of  this  year  a  22-hp.  gasoline  engine  was 
purchased  for  hauling.  Now  the  cars  are  heaped 
and  the  engine  hauls  them  directly  from  the  beds, 
thereby  giving  a  greater  daily  sludge  removal. 

Owing  to  the  adherence  of  sand  or  gravel  to  the 
under  side  of  the  sludge  when  the  latter  is  being  re¬ 
moved  the  4-in.  beds  need  a  renewal  of  the  sand 
layer.  A  small  amount  of  sand  has  been  added 
from  time  to  time,  but  it  is  fair  to  say  that  a  total 
of  3  in.  has  been  lost  by  all  beds  since  1915.  The 
adhesion  of  sand  or  gravel  to  the  sludge  is  a  matter 
of  considerable  importance  at  the  Baltimore  sewage 
works  because  the  sand-bed  dried  sludge  is  sold 
under  the  above-mentioned  five-year  contract  on  the 
basis  of  its  nitrogen  content.  If  the  sludge  after 
passing  through  this  company's  rotary  drier  con¬ 
tains  2  per  cent  nitrogen  as  equivalent  ammonia  on 
the  10  per  cent  moisture  basis,  the  city  receives 
81c.  per  ton,  weighed  with  the  water  content  rang¬ 
ing  from  10  to  15  per  cent.  There  have  been  times 
when  the  sludge  has  fallen  below  the  required  nitro¬ 
gen  content,  the  reason  for  which  can  be  readily 
seen  when  it  is  realized  that  the  mineral  matter, 
principally  sand,  has  run  as  high  as  39.6  per  cent. 
To  overcome  this  difficulty  one  of  the  beds  was 
partially  covered  with  a  slotted  wooden  platform. 
This  has  been  in  service  since  1916  and  is  still  in 
good  condition.  It  accomplished  the  purpose  for 
which  it  was  installed  and  the  sludge  disposed  of 
upon  it  has  acted  very  similar  to  that  placed  upon 
the  sand  direct.  The  present  contractor  for  the 
sand-bed  dried  sludge  does  not  seem  disposed  to 
renew  his  contract  on  the  basis  referred  to  above 


24  NINE  YEARS’  OPERATION  OF  THE 

because  he  claims  to  have  lost  money  during  the 
past  five  years.  Therefore  efforts  are  now  being 
made  to  form  a  new  contract. 

As  removed  from  the  beds  the  sludge  has  a  specific 
gravity  of  1.098  per  cent,  contains  70.67  per  cent 
water,  54.83  per  cent  volatile  matter,  and  2.264  per 
cent  nitrogen  on  the  dry  basis,  whereas  if  analyzed 
at  the  time  of  removal,  avoiding  the  sand,  the  figure.s 
are:  Specific  gravity,  1.077  per  cent;  water,  70.42; 
volatile  matter,  61.76;  nitrogen,  2.596  per  cent  on 
the  dry  basis. 

Although  the  drying  plant  is  not  a  part  of  the  sew¬ 
age  works  proper  regular  analyses  have  been  made 
of  the  moisture  and  nitrogen  content  of  the  sludge 
after  drying.  In  brief,  the  process  of  drying  is  to 
feed  the  wet  sludge  into  that  end  of  the  rotary  drier 
at  which  the  furnace  is  located.  The  heated  air 
and  sludge  pass  through  the  drier  together  and  dis¬ 
charge  into  a  dust  box.  From  here  the  sludge  passes 
to  an  elevated  revolving  screen  which  removes  match 
sticks,  corks,  etc.,  and  from  here  down  through  a 
chute,  at  the  end  of  which  it  is  bagged.  An  average 
of  all  the  analyses  made  upon  this  material  from 
April,  1916,  when  the  contract  started,  to  Dec.  31, 
1920,  is,  water  15.81  per  cent  and  nitrogen  on  the 
dry  basis  1.862  per  cent.  It  is  of  interest  to  know 
whether  there  is  any  appreciable  loss  of  nitrogen  in 
passing  through  the  drier.  For  this  purpose  a  run 
was  made  in  1916  with  the  following  result:  Before 
passing  through  the  drier  72.50  per  cent  water  and 
2.111  per  cent  nitrogen  on  dry  basis;  after  passing 
through  drier  18.89  per  cent  water  and  2.075  per 
cent  nitrogen  on  dry  basis. 

Costs  and  Revenue — Excluding  the  cost  of  grounds, 
consulting  engineers’  charges  and  a  proportion  of 
the  chief  engineer’s  salary,  the  total  construction 
cost  of  the  Baltimore  sewage  works  was  $2,500,000, 
in  round  numbers.  This  does  not  include  the  cost 
of  the  alterations  now  being  made  to  the  Imhoff 
tanks. 


BALTIMORE  SEWAGE  WORKS  25 

The  actual  expenditures  for  maintenance  and 
operation,  exclusive  of  interest  on  investment  and 
depreciation,  have  shown  a  gradual  decrease  in  the 
unit  cost  of  treatment  for  each  year  from  1912  to 
1917.  In  the  latter  year,  due  to  increased  costs  of 
material  and  labor,  this  price  rose.  Table  IV  and 
Fig.  2  show  the  average  and  maximum  daily  rate  of 
flow  for  each  year  as  well  as  the  cost  per  million 
gallons  for  treatment.  No  maximum  daily  rates  are 
available  previous  to  1914  and  it  might  be  said  that 
the  absolute  maximum  treated  has  been  106,090,000 
gallons. 

The  Imhoff  tanks  began  operating  in  August,  1915, 
and  continued  until  June,  1917.  They  were  idle,  as 
previouly  stated,  until  May,  1919,  when  two  were 
restarted  experimentally.  These  two  have  been  oper¬ 
ating  since  and  in  addition  16  others  were  added 
during  1920. 

The  comparative  cost  of  treating  sewage  in  the 
Imhoff  and  in  the  hydrolytic  tanks  with  sludge  di¬ 
gestion  in  separate  tanks  is  shown  by  Table  V. 

The  large  sum  of  $5,403  for  the  Imhoff  tanks  in 
1919  is  attributed  to  the  fact  that  but  two  tanks 
were  in  operation  and,  being  run  experimentally, 
had  more  time  applied  to  them  per  unit  volume  of 
sewage  treated  than  would  be  the  case  under  normal 
conditions.  The  additional  increase  for  1920  is 
caused  by  the  increases  given  the  laborers,  to  falling 
off  in  total  flow  and  the  necessity  of  disposing  of 
a  good  proportion  of  the  sludge  both  in  the  tanks 
and  on  the  sand  beds  which  had  resulted  from  the 
period  of  operation  in  1916  and  1917. 

The  cost  per  cubic  yard  of  sludge  removed  from 
the  sand  beds  with  an  average  haul  of  1,000  ft. 
was  $0,292  in  1915,  $0,292  in  1916,  $0,374  in  1917, 
$0,382  in  1918,  $0,454  in  1919,  and  $0,442  in  1920. 
In  .  1919,  with  the  electrically-driven  conveyor  load¬ 
ing  the  cars  at  the  digestion  tanks  and  conveying 
the  sludge  direct  to  the  dump,  with  a  haul  of  2,400 
ft.,  the  cost  was  $0,643  per  cubic  yard. 


26 


NINE  YEARS’  OPERATION  OP  THE 


TABLE  IV.  MILLIONS  OF  GALLONS  OF  BALTI¬ 
MORE  SEWAGE  TREATED  DAILY  AND  COST 
PER  MILLION  GALLONS. 


Year 

Average 

Daily 

Rate 

Average 
Maximum 
Daily  Rate 

Cost  of  Treatment 
per 

Million  Gallons 

1912 

11.80 

$4,860 

1913 

18.10 

4.800 

1914 

22.94 

32.96 

4.500 

1915 

31.80 

42.90 

3.450 

1916 

39.27 

52.10 

2.594 

1917 

44.38 

59.15 

2.882 

1918 

52.84 

68.48 

2.862 

1919 

55.06 

71.83 

3.389 

1920 

52.12^ 

69.97 

3.902^* 

♦The  falling  off  in  flow  is  due  to  the  decrease  in 
water  consumption  brought  about  by  a  campaign  for 
conservation  because  of  the  danger  of  a  water  short¬ 
age. 

♦♦The  increase  in  cost  is  due  to  the  increase  in  labor 
costs  and  decrease  in  sewage  flow. 


TABLE  V.  COST  PER  MILLION  GALLONS  OF 
TWO  METHODS  OF  TANK  TREATMENT 
AT  BALTIMORE. 


Hydrolytic  and 

Separate 

Year 

Imhoff  Tanks  Sludge  Digestion  Tanks 

1916 

$3,630 

$2,439 

1917 

3.933 

2.841 

1918 

Imhoff  Tanks  Not  in  Operation 

1919 

5.403 

3.337 

1920 

6.897 

3.738 

TABLE 

VI.  REVENUE 

FROM  SEWAGE  SLUDGE 

AT  BALTIMORE 

From  Drying  Plant 

Year 

From  Farmers 

(By  Contract) 

Total 

1914 

$1,931.33 

$1,931.33 

1915 

2,242.75 

2,242.75 

1916 

1,098.15 

$1,591.79 

2,689.94 

1917 

346.40 

28.07 

374.47 

1918 

344.80 

2,322.51 

2,667.31 

1919 

309.00 

1,557.61 

1,866.61 

1920 

233.00 

1,275.07 

1,508.07 

There  has  always  been  a  small  revenue  from 


rentals  of  land  and  buildings,  which  will  average  ap¬ 
proximately  $900  yearly. 

Since  1914  the  bar  screenings  and  sludge  have 
been  sold.  The  sludge  has  been  sold  both  wet  and 
dry;  that  is,  in  the  same  condition  as  when  placed  on 
the  sand  beds  and  as  taken  from  storage  piles  after 
being  removed  from  these  beds.  The  price  has  been 
25c.  per  load,  except  for  a  short  interval,  when  it 
sold  for  35c.  This  price  was  again  reduced  to  25c. 
per  load  after  a  falling  off  in  sales  proved  that  the 
farmers  were  unwilling  to  pay  the  10c.  increase. 


BALTIMORE  SEWAGE  WORKS  27 

The  wet  sludge  was  removed  in  tank  wagons  holding 
approximately  250  gal.,  whereas  the  dried  material 
has  been  sold  by  the  load,  one  to  two  tons  consti¬ 
tuting  a  load. 

In  1916  the  sale  of  sludge  to  farmers  was  cur¬ 
tailed  because  the  contract  for  sand-bed  dried  sludge 
began  in  the  spring  of  this  year.  In  1917  it  was 
still  further  curtailed  by  stopping  the  sale  of  wet 
sludge  to  those  farmers  who  hauled  over  the  main 
roads.  Table  VI  shows  revenue  from  the  sale  of 
sludge  to  the  farmers  and  under  contract. 

In  1917  the  revenue  dropped  exceptionally  low, 
due  to  the  fact  that  the  drying  plant  burned  down 
and  practically  all  of  the  sludge  which  was  dried  ran 
below  the  required  2  per  cent  equivalent  ammonia 
content  on  account  of  the  adhesion  of  sand  and 
gravel  in  removing  the  sludge  from  the  sand  beds. 
Of  the  total  output  of  this  drying  plant  approxi¬ 
mately  two-thirds  has  contained  the  required  amount 
of  nitrogen  and  has  therefore  been  paid  for. 


TABLE  VII  to  XI.  AVERAGE  MONTHLY  ANALY¬ 
SES  OF  BALTIMORE  SEWAGE  BEFORE,  DUR¬ 
ING  AND  AFTER  TREATMENT,  JANUARY, 

1912,  TO  DECEMBER,  1920,  INCLUSIVE. 

VII.  RAW  SEWAGE 


Chemical  Determinations 


in 

•rH 

m 

P 

o 

d 

m 

^  B 

o 

d  5 

• 

<u 

m 

•HQ 

bjo 

B  j- 

c, 

<o 

a 

in 

•r^ 

o  & 

0) 

Months 

m 

XJl 

mo 

January . .  .  . 

144 

89 

97 

February . . . 

164 

88 

94 

March . 

168 

88 

84 

April . 

182 

87 

93 

May . 

183 

134 

112 

June . 

181 

127 

128 

July . 

158 

110 

126 

August . 

223 

109 

125 

September.  . 

220 

78 

135 

October. . . . 

148 

80 

135 

November.  . 

168 

85 

125 

December.  . 

188 

94 

114 

.  P. 

M. 

Bacteria 

per  C.  C. 

Cj  ® 

m 

Plain 

be 

c 

B 

in 

®  <u 

d 

a> 

c 

u 

0) 

u 

-M 

otal  c 
Agar 

'd 

o 

380,000 

41,500 

850,000 

47,000 

550,000 

37,500 

900,000 

66,000 

1,400,000 

190,000 

1,800,000 

180,000 

2,200,000 

270,000 

2,600,000 

290,000 

4,100,000 

650,000 

2,100,000 

250,000 

1,200,000 

220,000 

•  • 

460,000 

72,000 

28  NINE  YEARS’  OPERATION  OF  THE 


TABLE  VIII.  EFFLUENT  FROM  HYDROLYTIC 

TANKS 


January.  ... 

80 

26 

95  .  . 

210,000 

35,000 

February. . . 

83 

33 

107  .  . 

190,000 

29,000 

March . 

67 

36 

Ill  .  . 

210,000 

33,000 

April . 

69 

31 

106  .  . 

350,000 

48,000 

May . 

59 

44 

120 

750,000 

100,000 

June . 

109 

60 

120 

.  .  1,100,000 

73,000 

July . 

107 

58 

124 

.  .  1,300,000 

110,000 

August . 

104 

49 

100 

.  .  1,000,000 

150,000 

September.  . 

63 

37 

109 

.  .  1,700,000 

110,000 

October.  .  .  . 

70 

36 

120 

.  .  2,000,000 

100,000 

November.  . 

69 

34 

116 

600,000 

87,000 

December.  . 

65 

35 

97 

390,000 

45,000 

TABLE  IX.  EFFLUENT  FROM  IMHOFF  TANKS 


January . 

31 

105 

320,000 

61,000 

February. . . 

36 

115 

330,000 

56,000 

March . 

26 

134 

150,000 

51,000 

April . 

23 

144 

750,000 

100,000 

May . 

25 

146 

1,100,000 

210,000 

June . 

25 

150 

800,000 

120.000 

July . 

23 

August . 

24 

•  ••••••• 

••••••• 

September.  . 

57 

•  ••••••• 

•  •••••• 

October .... 

20 

•  ••••••• 

November.  . 

23 

•  • 

December. .  . 

41 

•  •  • 

•  r 

•  • 

•  ••••••• 

•  •  •  •  w  «> 

TABLE  X. 

EFFLUENT 

FROM  FILTER  BEDS 

January. ..  .  . 

60 

56 

37 

5.2 

81 

53,000 

5,800 

February.  .. 

73 

46 

40 

5.4 

82 

140,000 

9,900 

March . 

63 

45 

35 

6.8 

88 

49,000 

5,600 

April . 

93 

67 

34 

8.0 

94 

140,000 

9,300 

May . 

67 

40 

29 

11.3 

95 

260,000 

18,500 

June . 

96 

52 

32 

8.7 

92 

700,000 

23,000 

July - ... 

65 

52 

36 

8.1 

91 

440,000 

31.000 

August . 

58 

44 

26 

8.0 

96 

470,000 

35,000 

September.  . 

37 

29 

30 

6.5 

93 

650,000 

75,000 

October . 

48 

32 

30 

6.7 

98 

270,000 

28,000 

November.  . 

59 

44 

36 

7.6 

96 

140,000 

17,000 

December.  . 

96 

54 

40 

6.1 

83 

88,000 

14,000 

TABLE 

XI. 

FINAL 

EFFLUENT 

January. .  .  . 

51 

23 

28 

5.4 

85 

59,000 

6,700 

February. .  . 

32 

22 

29 

5.4 

88 

79,000 

5,700 

March . 

33 

20 

28 

6.8 

93 

59,000 

5,000 

April . 

44 

29 

23 

7.2 

90 

110,000 

8,200 

May . 

53 

33 

22 

8.9 

93 

600,000 

14,000 

June . 

49 

32 

24 

7.4 

97 

900,000 

15,000 

July . 

39 

32 

21 

7.4 

94 

1,300,000 

32,000 

August . 

38 

21 

16 

7.1 

98 

900,000 

29,000 

September.  . 

26 

16 

18 

6.1 

98 

850,000 

68,000 

October.  .  .  . 

28 

17 

20 

6.4 

99 

600,000 

20,000 

November.  . 

39 

24 

24 

7.2 

98 

350,000 

15,000 

December. .  . 

40 

29 

29 

5.9 

86 

200,000 

15,000 

Plant  Performance — Practically  no  mention  has 
been  made  thus  far  of  the  analytical  results  of  the 
sewage  at  its  different  stages  of  transit  through 


the  plant.  While  the  routine  analyses  do  not  in¬ 
clude  total  solids,  chlorine,  alkalinity  and  fats,  an 
average  for  these  is  773,  161,  153  and  81.7  p.  p.  m. 
respectively.  The  figure  for  total  solids  was  ob- 


29 


BALTIMORE  SEWAGE  WORKS 

tained  in  1916,  at  which  time  the  suspended  solids 
were  138  p.  p.  m.  A  24-hr.  run  in  February,  1920, 
gave  an  average  of  ,17.8  p.  p.  m.  free  ammonia. 

To  acquire  some  knowledge  of  the  performance 
of  the  plant  during  the  various  seasons  the  aver¬ 
age  of  all  results  for  the  different  months  from 
1912  through  1920  inclusive  have  been  compiled 
and  are  presented  herewith  in  Tables  VII  to  XI. 
The  yearly  averages  are  given  in  Tables  XII  to  XV. 
Table  XVI  presents  the  plant  efficiency  by  showing 
the  per  cent  reduction  in  solids,  bacteria,  and  de- 
oxygenating  powers  between  the  raw  sewage  and 
final  effluent  and  by  presenting  the  nitrates  and 
relative  stability  of  the  final  effluent,  the  raw  sew¬ 
age  being  practically  devoid  of  oxygen  in  any 
form. 

The  final  effluent  of  the  plant  during  1919  con¬ 
tained  2.50  p.  p.  m.  dissolved  oxygen  and  had  an 
oxygen  consuming  value  of  25.3  p.  p.  m.  by  the 
permanganate  method  with  a  30-minute  digestion 
period.  During  1916  the  total  solids  were  616 
p.  p.  m.  and  the  suspended  solids  47  p.  p.  m. 

Samples  of  the  river  water  are  taken  at  fairly 
regular  periods  from  April  to  December  of  each 
year.  There  are  eleven  stations,  one  approximately 
one-half  mile  up  stream  from  the  discharge  pipe, 
another  at  the  mouth  of  the  discharge  pipe,  and  the 
others  down  stream  with  a  maximum  distance  of 
9%  miles  below  the  outlet.  The  nearest  oyster 
beds  are  14  miles  below  the  discharge.  At  all 
times  since  the  plant  operation  began  there  has 
been  a  plentiful  supply  of  oxygen  at  each  station. 

This  summary  would  not  be  complete  without 
some  mention  being  made  of  the  two  most  im¬ 
portant  experiments  conducted  at  the  disposal  plant 
since  its  operations  began,  on  activated  sludge  and 
grease  recovery. 

Activated  Sludge — Under  an  agreement  between 
the  Sewerage  Commission  of  Baltimore  and  the  U.  S. 


30  NINE  YEAPwS’  OPERATION  OF  THE 

Public  Health  Service  to  work  jointly  on  the  ac¬ 
tivated  sludge  process  a  series  of  laboratory  experi¬ 
ments  was  started  early  in  March,  1915,  and  con¬ 
tinued  until  August  of  the  same  year.  There  were 
six  experiments  conducted  simultaneously,  five  of 
which  were  with  the  air  drawn  through  the  bottle 
by  means  of  an  aspirator,  while  in  the  sixth  the 
aerating  was  done  by  means  of  a  propeller  inclosed 
in  a  tube.  The  continuous  operation  of  the  latter 
experiment  was  broken  up  so  often  that  reliable 
results  were  not  obtained.  The  other  five,  run 
under  different  conditions,  indicated  that  the  ac¬ 
tivated-sludge  process  conducted  under  the  fill- 
and-draw  method  was  applicable  to  Baltimore  sew¬ 
age;  that  it  was  possible  to  generate  sludge  more 
quickly  with  mixtures  containing  final  settling  basin 
sludge  than  with  raw  sewage  alone  or  raw  sewage 
plus  raw  sewage  sludge,  and  that  the  absence  of 
light  had  no  noticeable  effect  upon  the  activated- 
sludge  organisms. 

TABLES  XII  TO  XV.  AVERAGE  YEARLY  ANALY¬ 
SES  OP  BALTIMORE  SEWAGE  BEFORE,  DUR¬ 
ING  AND  AFTER  TREATMENT,  1912  TO 

1920,  INCLUSIVE. 

TABLE  XII.  RAW  SEWAGE 
Chemical  Determinations,  P.  P.  M.  Bacteria  per  C.  C. 


w 

>> 

+-> 

O 

m 

d 

d 

-hS 

(rf 

d 

ci 

bfi 

d 

'd 

<X) 

'd 

<D 

ft 

xn 

ri 

Q 

m 

bfi 

d 

Biochemica 
Oxygen  De 

CO 

0) 

-4-» 

ci 

u 

Relative  St 
Per  Cent 

Total  on  P 
Agar 

s 

u 

o 

'd 

Year 

Xfl 

m 

w 

1912 . 

54 

•  •  • 

•  •  • 

*1,900,000 

56,000 

1913 . 

122 

•  •  • 

115 

1,700,000 

360,000 

1914 . 

120 

•  •  • 

131 

2,300,000 

150,000 

1915 . 

256 

183 

113 

1,100,000 

160,000 

1916 . 

138 

102 

91 

1,100,000 

89,000 

1917 . 

260 

69 

90 

950.000 

59,000 

1918 . 

287 

49 

127 

800,000 

40,500 

1919 . 

111 

66 

171 

1,400,000 

160,000 

1920 . 

172 

120 

•  •  • 

•  • 

2,200,000 

320,000 

♦Gelatin 

Count 

at  20  Degrees 

C. 

BALTIMORE  SEWAGE  WORKS 


31 


TABLE  XIII.  EFFLUENT  FROM  HYDROLYTIC 

AND  IMHOFF  TANKS 


1912 . 

.  31 

•  •  • 

•  •  • 

•  ••••••• 

•  •••••* 

1913 . 

.  76 

•  •  • 

129 

1,300,000 

310,000 

1914 . 

.  85 

•  •  • 

190 

3,300,000 

230,000 

1915 . 

.  109 

58 

153 

1,100,000 

210,000 

1915 . 

*55 

•  •  • 

•  ••••••• 

•  •••••• 

1916 . 

35 

118 

800,000 

94,000 

1916 . 

*26 

*132 

*550,000 

*100,000 

1917 . 

34 

135 

850,000 

70,000 

1917 . 

*21 

•  •  • 

•  ••••••• 

•  •••••• 

1918 . 

36 

114 

850,000 

63,000 

1919 . 

37 

93 

700,000 

66,000 

1920 . 

39 

92 

650,000 

93,000 

1920 . 

♦Imhoff 

....  *28 

Results. 

•  •  • 

•  ••••••• 

••••••• 

TABLE 
1912 . 

XIV. 

22 

EFFLUENT  FROM 
.  8  3  98 

FILTER 

BEDS 

1913 . 

.  56 

•  •  • 

30 

8.2 

91 

180,000 

36,500 

1914 . 

.  60 

•  •  • 

41 

7.2 

78 

470,000 

35,000 

1915 . 

.  89 

68 

37 

6.2 

87 

250,000 

31,000 

1916 . 

48 

33 

4.4 

93 

290,000 

12,000 

1917 . 

49 

38 

7.4 

92 

200,000 

12,000 

1918 . 

40 

32 

8.1 

96 

160,000 

7,700 

1919 . 

46 

31 

7.7 

89 

190,000 

9,300 

1920 . 

1912 . 

.  ...  27  27  7.0 

TABLE  XV.  FINAL 

1 .3  .  7  5 

92  150,000 

EFFLUENT 

98  _ 

13,000 

2,400 

16,000 

1913 . 

.  22 

•  •  • 

13 

7.9 

99 

260,000 

1914 . 

.  27 

•  •  • 

26 

6.9 

92 

700,000 

24,000 

1915 . 

.  43 

28 

21 

5.5 

94 

750,000 

25,000 

1916 . 

.  47 

31 

22 

4.1 

92 

750,000 

11,500 

1917 . 

.  42 

27 

28 

7.4 

92 

330,000 

11,500 

1918 . 

.  46 

25 

26 

7.6 

95 

500,000 

8,500 

1919 . 

.  43 

24 

24 

6.7 

92 

250,000 

13,500 

1920 . 

.  41 

18 

25 

6.5 

90 

230,000 

19,000 

TABLE  XVI.  AVERAGE  YEARLY  EFFICIENCY 
RESULTS  BALTIMORE  SEWAGE-WORKS,  1912 
TO  1920  EXPRESSED  AS  PERCENTAGES 
FROM  RAW  SEWAGE  TO  FINAL 
EFFLUENT 

- Percentage  Reduction - -  — Increase  in — 

Biochem-  Relative 

Sus-  ical  Acid  Stability 


pended  Settling  Oxygen  Forming  Nitrates 


Year 

Solids 

Solids 

Demand  Bacteria  p. 

p.  m.  Per  Cent 

1912 

75.5 

•  •  •  • 

•  •  •  • 

95.8 

7.5 

98 

1913 

82.3 

•  •  •  • 

88.9 

95.6 

7.9 

99 

1914 

69.0 

•  •  •  • 

80.0 

84.2 

6.9 

92 

1915 

83.2 

84.7 

81.4 

84.4 

5.5 

94 

1916 

65.9 

69.6 

75.3 

87.1 

4.1 

92 

1917 

83.9 

61.4 

68.9 

80.6 

7.4 

92 

1918 

84.7 

50.4 

61.2 

79.0 

7.6 

95 

1919 

61.3 

64.3 

81.2 

91.6 

6.7 

92 

1920 

76.2 

85.1 

85.5 

94.1 

6.5 

90 

Averages  75.8  69.3  77.8  88.0  6.7  94 

Note:  Raw  Sewage  practically  devoid  of  oxygen  in 
any  form. 

The  large-scale  experiment  in  one  of  the  Imhoff 
tanks,  altered  for  the  purpose,  was  begun  in  the 


82  NINE  YEARS’  OPERATION  OF  THE 

early  part  of  1915.  The  intention  was  to  gain  in¬ 
formation  as  to  the  applicability  and  costs  of  the 
continuous-flow  process.  Although  it  was  possible 
to  get  very  well  ^^activated’’  sludge  with  a  sludge  ' 
ratio  as  high  as  30  per  cent,  a  short  time  after  a 
continuous  flow  of  raw  sewage  was  allowed  to  pass 
through  the  tank  the  sludgei  lost  its  oxidizing 
power.  A  number  of  changes  were  made  because 
of  mechanical  difficulties  and  clogging  of  the  air 
distributors.  Finally,  the  best  results  were  obtained 
by  the  use  of  the  filtros  disk  grid.  Unfortunately  a 
fire  destroyed  the  air  compressor  house  on  Feb.  15, 
1916,  burning  practically  everything,  including  the 
manometer  for  measuring  the  volume  of  air  used. 
After  this  building  was  rebuilt  experiments  were 
started  with  another  type  of  blower.  This  blower 
did  not  perform  satisfactorily  and  in  consequence 
the  laboratory  analyses  were  discontinued  on  May  19, 
1916.  Efforts  to  adjust  the  blower  difficulties  contin¬ 
ued  until  the  fall  of  1916,  when  the  experiment  was 
stopped.  There  is  no  account  of  the  exact  reason 
for  discontinuing  the  activated-sludge  experiments, 
but  the  most  reliable  information  is  to  the  effect 
that  the  lack  of  funds  was  the  cause.  More  ex¬ 
tensive  data  on  the  Baltimore  experiments  can  be 
found  in  Engineering  Record,  April  24,  1915,  p. 
521,  and  July  3,  1915,  p.  23;  Engineering  News, 
July  22,  1915,  p.  164,  April  27,  1916,  p.  798,  and 
July  20,  1916,  p.  106,  and  the  Municipal  Journal  and 
Engineer,  Oct.  19,  1916. 

Grease  Recovery — Beginning  in  June,  1915,  and 
continuing  approximately  five  months,  an  investi¬ 
gation  was  conducted  at  the  Baltimore  sewage- 
works  by  G.  J.  Requardt,  formerly  acting  division 
engineer,  and  Arthur  B.  Morrill,  formerly  chemist- 
bacteriologist,  to  determine  the  feasibility  of  ex¬ 
tracting  fats  and  grease  from  the  city  sewage. 

The  large-scale  experiment  consisted  of  pump¬ 
ing  the  sewage  continuously  from  the  outfall  sewer 
into  a  cylindrical  tank  of  428  gal.  capacity  at  the 


BALTIMORE  SEWAGE  WORKS 


33 


rate  of  approximately  a  5-hr.  detention  period. 
Enough  sulphuric  acid  was  constantly  added  to  the 
inflowing  sewage  so  that  the  effluent  was  always  acid, 
A  number  of  small-scale  experiments,  somewhat  simi¬ 
lar  to  the  tank  experiment,  were  conducted  in  the 
laboratory.  In  the  latter  enough  sulphuric  acid 
was  added  to  the  sewage  so  that  it  remained  acid 
after  3-hr.  settling. 

The  results  of  these  experiments  show  that  the 
value  of  30.9  grams  per  capita  is  reliable  for  the 
total  amount  of  grease  in  Baltimore  sewage,  that 
the  sludge  accumulated  in  the  large  tank  experi¬ 
ment  contained  on  the  dry  basis  27.5  per  cent  of 
grease,  of  which  86.34  per  cent  was  saponifiable 
and  13.66  per  cent  unsaponifiable.  The  sludge  ob¬ 
tained  had  a  very  objectionable  odor,  contained 
93.9  per  cent  water  and  dried  on  the  sand  bed 
much  less  readily  than  good  non-acidified  sludge. 
Even  after  48  days  drying  it  retained  66.8  per  cent 
moisture. 

At  the  conclusion  of  this  work  correspondence 
with  six  different  manufacturers  revealed  the  fact 
that  only  one  would  consider  the  use  of  the  grease 
produced,  and  his  requirements  as  to  purity  and 
per  cent  of  water  made  the  cost  prohibitive. 

The  mechanical  recovery  of  the  floating  grease 
by  skimming  and  then  applying  heat  and  straining 
produced  a  substance  with  a  distinct  sewage  odor 
and  with  such  an  amount  of  water  that  it  was 
practically  impossible  to  find  a  market  for  it.  For 
this  reason  nothing  further  has  been  done  along 
these  lines. 

When  the  work  of  The  Sewerage  Commission  end¬ 
ed  in  the  early  part  of  1916  the  sewage- works  was 
put  under  the  supervision  of  the  Highways  En¬ 
gineer's  Department,  of  which  A.  E.  Christhilf  is  the 
present  head,  with  Milton  J.  Ruark  in  direct  charge 
of  the  sewer  division,  and  J.  W.  Holden  superintend¬ 
ent  of  the  plant. 


